Collaborative Research: Interaction Between Plumes and Surface Waves from the Surf Zone to the Inner-Shelf

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Rivers play an important role in delivering land-derived material (e.g., sediment, contaminants) and freshwater to the coastal ocean, the latter impacting coastal circulation. Yet the role of near-shore waves and wave breaking in plume mixing, transport, and ultimate fate along the coastline is not yet understood. To address this knowledge gap, this project uses a combination of measurements and numerical models to determine the fate of buoyant river plume water in the coastal ocean. Observations from two field experiments will measure river plume spreading and mixing in the coastal ocean. Realistic simulations of these field sites combined with idealized numerical simulations will allow the experiment to span a broad range of parameter space in order to be relevant to different types of rivers across a variety of coastlines. The improved understanding of river water mixing and fate in the coastal ocean that will result from the work is crucial to quantifying the spreading of sediment, pollutants, larvae, and other important material in the nearshore. Thus, this work will aid in identifying the spatial and temporal scales relevant to mitigating risks due to poor coastal water quality under varying conditions, improving coastal water quality predictions and reducing human exposure to contaminated waters. This study will improve the dynamical understanding of small Kelvin number, small-moderate discharge river plumes in the presence of depth-limited surface waves. Prior research has shown important wave impacts on plume structure and mixing, yet significant knowledge gaps remain in the understanding of wave/plume interactions across a range of river discharge and across the surf zone/inner-shelf region. These knowledge gaps limit our ability to predict the ultimate fate of small rivers and the material they carry with them, relevant to coastal pollution, larval transport, and more. Preliminary simulations suggest there is significant wave-driven plume modification including enhanced along-shore spreading within the surf zone and decreased freshwater export to the inner-shelf. A suite of idealized coupled ocean circulation and wave propagation model simulations for varying river discharge, wave height, wave direction, tidal stage, and Coriolis forcing will provide a parameter space to explore wave/plume interaction. The results from idealized studies will be augmented with conclusions from an existing study site (with existing observations and realistic model output) and a new complementary study site. The observational campaign and realistic simulations of the new study site are designed to capture the missing parameter space and plume metrics that were not captured at the existing site. Finally, combined analyses of the suite of idealized and realistic models and observational datasets will be done to address four key hypotheses. Ultimately this study will: (1) Determine the physical mechanisms that trap plume water within the surf zone, (2) characterize the role of riverine versus wave momentum fluxes on plume extent, (3) identify pathways through which freshwater trapped in the surf zone gets exported to the inner shelf, and (4) test existing turbulence closure models in the region of depth-limited wave breaking in the presence of buoyancy gradients.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.